In the field of RF power amplification, with the increasing complexity of signals to be amplified (such as GSM, WCDMA, LTE, etc. . . . ) and the demand for efficiency, the preferred mode of operation of a RF transistor is a common source (emitter) class AB linear amplifier.
In order to maintain linearity, it is known to use a bias circuit to provide thermal compensation. This has been implemented in MMIC's using the well-known current mirror arrangement of transistors. When integrated on the same die as a RF power transistor, this provides thermal tracking and process compensation.
With the continued drive to further increase efficiency and reduce the size and cost of amplifiers, Doherty amplifiers are now widely used because of their advantages in efficiency in back-off. In its simplest configuration, a Doherty amplifier consists of two amplifiers, namely a main (or carrier) amplifier and an auxiliary (or peaking amplifier) connected in parallel with their outputs joined by a quarter-wave transmission line. The main amplifier is typically a Class B or Class AB type linear power amplifier and the auxiliary amplifier is typically a Class C type non-linear power amplifier.
Thus, to be suitable for Doherty amplifier, a bias circuit has to be able to provide a Class C bias level, which corresponds to a class AB level from which a fixed voltage is subtracted. A conventional current mirror (like that shown in FIG. 1 for example) is therefore not suitable for Class C.